One conventional mode of use of silica sol is incorporation into a resin or resin material to thereby provide a curable resin composition. In such a case, silica sol is used for improving properties of resin products such as surface hardness, shrinkage after curing, thermal expansion property, heat resistance, and insulation performance. Silica sol is also used in combination with silica powder having a micron-order to a submicron-order particle size, the two silica components being incorporated into a resin so as to enhance dispersibility of the silica powder in resin and silica filling density, to thereby enhance performance of the resin products.
Hitherto, for years there has been known a method for producing silica sol for use in such a silica-containing resin composition, the method including neutralization or ion-exchange of water glass serving as a raw material. Also, it has been known that silica micropowder can be produced through pyrolysis of silicon tetrachloride. An alternative known method for producing a silica sol is based on hydrolysis of a silicon alkoxide in an alcoholic aqueous solution in the presence of a basic catalyst. In one reported method, a 0.28 mol/L tetraethyl silicate is added to an alcoholic solution containing several mol/L-order ammonia and several mol/L to 15 mol/L water, and the mixture is hydrolyzed, to thereby obtain silica particles having a diameter of 50 to 900 nm (see, for example, Non-Patent Document 1).
There has been also disclosed a method for producing a hydrophilic organic-solvent-dispersed, neutral silica sol having a metallic impurity level of 1.0 ppm or lower, the method including a step of hydrolyzing an alkoxysilane in an aqueous alcoholic solution (see, for example, Patent Document 1). Another disclosed silica sol production method includes hydrolysis of tetraethyl silicate in the presence of sodium hydroxide or a water-soluble amine as a hydrolysis catalyst (see, for example, Patent Document 2).